help with audio resesrch d76

Larry, hopefully it doesn't have the white relay in it either. That can become problematic.

Hifigeek1 I need your help. What was the purpose of the "Servo" circuit? in the ARC 90B? Also, is the white/gray relay located down in between the large grey tall power supply caps? where the 6550 power supply tube was located in the 90? I do not have one in mine. The reason I am asking is that all of a sudden my 90B(ish) upon start up, the output tubes glow orange after about 30 seconds and then the fuse blows. Techs look at my 90B and say it does not match the ARC schematic for 90B?? Again ARC said it started out as a 90?? or 90Revised. Very confused. Thanks Larry

Larry, In the D90, V19 is the 6550 regulator. In the D90B the regulator consists of a 12BH7 V15, and a 12AT7 V16. The B version does not use a 6550 for voltage regulation. It is possible the unit started out as a D90 and got updated to D90B. I have seen updates like this before. You unit needs to be serviced by someone who knows ARC products. I would be happy to take care of it for you if you do not wish to ship it back to ARC. The servo was used to illuminate coupling caps.

HiFigeek1 you have sent myself and my fellow audio buddies on a wild goose chase.....for several days...we just thought we were dumb and didn't know how to ask you...Illuminate coupling caps???? We thought you knew an audio term we did not know....WELL DID YOU MEAN ELIMINATE coupling caps...??? Larry ;)

In certain situations a servo ic is used to eliminate coupling caps from one section of a given circuit, utilizing the ic in a feedback loop, usually at the input. Although coupling caps are a necessity in tube products, they do have their own set of coloration that add to the sound of the product. A tone control of sorts. Some designers felt that by removing them at the input of the circuit and replacing them with a servo ic there would be less coloration. What we now know is there's no free lunch. Servo ic's do have their own set of coloration with each manufacturers ic's sounding different. In some cases they can actually sound worse than a high quality coupling cap.